Senile cataract is a major cause of blindness and visual disability throughout the world. Despite its high frequency and public health impact, the factors predisposing to cataract development are largely unknow. The first aim of this case-control study is to evaluate associations of risk factors with each major type of senile cataract. The study will a) evaluate risk factors for nuclear, cortical, nuclear-cortical, and posterior subcapsular cataract, and b) determine whether these types of cataract differ with regard to their risk factors. The major factors under investigation include nutritional status (as measured by biochemical, dietary, and anthropometric assessment), systemic diseases, and history of sunlight exposure. Ocular factors, family history, demographic, and other variables will also be evaluated. Four case groups and one control group of 300 patients each will be entered into the study. The four case groups consist of patients with nuclear, cortical, nuclear-cortical, or posterior subcapsular cataracts, as determined by ophthalmologic examination and lens photography. All participants will have an interview, and blood will be drawn for biochemical assessment. Medical history will be validated by contacting the patient's physician. The second aim of the study is to evaluate methods of in vivo cataract classification. The study will compare classification by ophthalmologic examination to classification by photographic methods. In the subset of cataract patients who will have intracapsular cataract surgery, the study will also compare in vivo and in vitro classifications. The third aim is to compare the findings of the study with those of a parallel study being conducted in India. This comparison may identify some reasons for the large differences in cataract prevalence in both areas. By accomplishing these objectives, the study has the potential of making an important contribution to our understanding of cataract etiology and, eventually, to the development of possible preventive measures.